Conventionally, it is known that Au—Sn alloy solder pastes are used for bonding semiconductor devices (e.g., GaAs device, high frequency GaAs device, heat transfer device, or the like) to substrates, or used for package sealing of SAW filters, crystal radiators or the like, which requires extreme fineness and air-tightness. It is also known that an Au—Sn alloy powder contained in an Au—Sn alloy solder paste is a eutectic alloy polymer having a composition consisting of Sn of 20% by mass and a remainder containing Au and inevitable impurities, and that such an Au—Sn alloy powder can be obtained by gas atomization.
In recent years, minimization of package size has rapidly progressed. Specifically, a package to be sealed has been minimized to a size of 1.6 mm (length)×1.0 mm (width), and it is attempted to further minimize the size. With respect to the frame of the lid-bonding portion provided in a package having a size of 1.6 mm (length)×1.0 mm (width), the width thereof is as small as from 100 to 250 μm, and sealing of the package is performed by applying the above-mentioned Au—Sn alloy solder paste to this narrow frame, followed by placing a lid thereon and heating. As the minimization of package size progresses, the width of the frame becomes increasingly smaller. In the case where package sealing of a SAW filter, a crystal radiator or the like (which requires a high level of air-tightness) is performed by soldering on such a narrow frame, when a large amount of voids are generated at the soldering portion, air-tightness gets lowered, thereby leading to lowering of soldering reliability. Especially, generation of large voids having a diameter of more than 25 μm is one of the main reasons for the lowering of air-tightness in small packages.
For solving this problem, there has been provided an Au—Sn alloy solder paste by using an Au—Sn alloy powder having a composition consisting of Sn of 15-25% by mass and a remainder containing Au and inevitable impurities, in which the surface of the Au—Sn alloy powder has a composition consisting of Sn of 7-12% by mass and a remainder containing Au and inevitable impurities, and has Au-rich primary crystal phase regions of 10 area% or more crystallized. This Au—Sn alloy powder which has Au-rich primary crystal phase regions of 10 area% or more crystallized in the surface is obtained by gas atomizing a molten Au—Sn alloy under conditions in which the molten Au—Sn alloy is maintained at a temperature lower than a typical molten temperature (i.e., 300° C. to lower than 600° C.), the molten metal pressurizing pressure and the injection pressure are respectively maintained at 20 kPa to lower than 300 kPa and 500 kPa to lower than 5,000 kPa (which are lower than typical pressure conditions), and the nozzle diameter is 0.3 mm to less than 2 mm. In the surface portion of the thus obtained Au—Sn alloy powder, the proportion of Au-rich primary crystal phase regions crystallized is large as compared to the composition of the entire Au—Sn alloy powder. Therefore, this Au—Sn alloy powder is advantageous in that the amount of surface oxidation becomes small, in that the surface is not easily flawed during classification by sieving, and in that a paste containing this Au—Sn alloy powder has small generation of voids, thereby rendering it possible to achieve a good bonding (see 1: Japanese Unexamined Patent Application, First Publication No 2003-105462 (“JP ′462”).